The Boston Marathon Bombings of 2013 , The 2013 Boston Marathon was marred by tragedy when a pair of homemade bombs detonated in a crowd of spectators, killing 3 people and injuring more than 260. The attack took place on April 15 at approximately 2:50 pm and occurred a short distance from the finish line.

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Smoke from the first explosion clears as a second detonation can be seen (background) in the bomb …

David L. Ryan—The Boston Globe/AP

The Attacks

The marathon is traditionally held on Patriots’ Day, a public holiday in Massachusetts that commemorates the American Revolutionary War Battles of Lexington and Concord. The festive atmosphere draws hundreds of thousands of spectators to the 26-mi 385-yd route (the standard marathon distance) from Hopkinton, Mass., to Boston’s Back Bay neighbourhood. More than 26,000 runners participated in the race in 2013, marking the 117th time that the world’s oldest annual marathon had been contested. About five hours into the race, the first bomb exploded less than half a block from the finish line, on the north side of Boylston Street. Roughly 12 seconds later a second bomb exploded some 180 m (600 ft) from the first. It too was planted on the north side of Boylston Street amid a crowd of onlookers. First responders reacted immediately, and a medical tent that had been erected to treat runners was turned into an emergency medical facility. Three bombing victims died of their injuries, and more than 100 of the seriously injured were transferred to area hospitals as local police and federal investigators surveyed a crime scene that covered 15 square blocks.

In the days following the attacks, law-enforcement personnel solicited assistance from the public, asking for photographs or video footage that might prove relevant to their investigation. It was revealed that devices used in the attacks were household pressure cookers that had been packed with an explosive substance, nails, and ball bearings—the latter two elements acting as shrapnel when the bombs detonated. On April 18 the FBI released images and video of two men identified as suspects in the attacks, including one photograph that showed one of the men placing a package at the location of the second explosion.

The Manhunt

Within hours the fatal shooting of an MIT campus police officer in Cambridge, Mass., and the armed carjacking of a sport-utility vehicle in Boston’s Allston neighbourhood spurred speculation about a possible connection between these crimes and the marathon bombings. Police pursued the stolen vehicle to the Boston suburb of Watertown, and an intense firefight ensued. Improvised explosive devices were thrown at the police; as many as 300 rounds were exchanged; and a police officer was shot and wounded. During the gun battle, 26-year-old Tamerlan Tsarnaev, identified as one of the two suspects in the bombing, was seriously wounded by explosives and multiple gunshots. He was apprehended by police, but he was further injured when the second suspect—his younger brother, Dzhokhar Tsarnaev—struck him with a car as he fled the scene. Tamerlan Tsarnaev was taken to a hospital and was pronounced dead in the early morning hours of April 19.

In an image taken by the Massachusetts State Police, Dzhokhar Tsarnaev, one of the two suspects in …

Sean Murphy—Massachusetts State Police/AP Images

With one suspect dead and another on the run, police launched a massive house-to-house manhunt that covered the surrounding area. Much of Greater Boston came to an unprecedented standstill as officials requested that residents remain in their homes and that businesses not open. The “stay home” order was lifted at 6:00 pm on April 19, and a Watertown resident whose house was several blocks from the shoot-out went to his backyard to check on the boat that was parked there. He noticed that its protective tarp had come loose and, while adjusting it, he observed Dzhokhar Tsarnaev hiding in the boat. Police soon arrived, and after a short standoff, Tsarnaev was taken into custody. On April 22 federal prosecutors charged him with having used a weapon of mass destruction in the marathon attacks; if convicted, he faced the possibility of the death penalty.

In the weeks and months that followed, investigators attempted to understand the two brothers’ motives and determine whether they had ties to larger, organized terrorist networks. Many also posed a question that was articulated by Pres. Barack Obama during an address to the nation; “Why did young men who grew up and studied here, as part of our communities and our country, resort to such violence?”

Dzhokhar Tsarnaev had arrived in the United States with his parents in 2002, and Tamerlan joined them a year later. Prior to their immigration to the U.S., the ethnic Chechen family had lived in Kyrgyzstan and the Russian republic of Dagestan. The elder Tsarnaev was an aspiring boxer, and he had had some successes at the amateur level, but he began to exhibit signs of Islamic radicalization in 2009. In 2011, acting on a request from the Russian government, the FBI investigated Tamerlan, but it found no evidence of terrorist activity. The following year Tamerlan spent six months in Dagestan. Upon his return he created a channel on the Web site YouTube with links to a number of extremist videos and applied for American citizenship.

Dzhokhar, meanwhile, was a popular student at the University of Massachusetts Dartmouth. When three of his friends learned of his role in the bombing, they gathered at his dormitory room and discovered a backpack that contained several fireworks that had been emptied of powder. The trio threw away the backpack and its contents (it was subsequently recovered), an action that led to their being charged with an assortment of charges related to hindering or misleading investigators.

Although there appeared to be no concrete links between the Tsarnaev brothers and any larger plots or terrorist groups, Dzhokhar revealed to investigators that he and his brother had obtained the plans for the bombs from Inspire, an online newsletter published by al-Qaeda in the Arabian Peninsula (AQAP). This led analysts who were following the investigation to conclude that the Tsarnaevs were “self-radicalized,” having developed a personal militant ideology that drew from disparate sources without being directly connected to any of them. Investigators uncovered gruesome evidence of the violent nature of that ideology when Tamerlan was implicated in a triple homicide that took place in Waltham, Mass., in September 2011. Ibragim Todashev, an associate of Tamerlan’s, also confessed to his role in the murders before he was shot and killed during an interrogation by the FBI.